if we create a grid with default selection model then this model will be created with allowDeselect config with default value 'false' (see Ext.Selection.Model class). That is why the following code in getSelectionModel function of parent to grid Ext.Panel.Table class does not work:

Code:

Ext.applyIf(selModel, {
allowDeselect: me.allowDeselect
});

As you can see the applyIf function will never work and seems to be replaced with Ext.apply function: